1. Field of Invention
This invention relates to wireless communications systems. Specifically, the present invention relates to receivers for demodulating quick paging channels in communications systems employing more than one paging channel to facilitate offline processing.
2. Description of the Related Art
Wireless communications systems are employed in a variety of demanding applications ranging from search and rescue to Internet applications. Such applications require reliable, cost-effective, and space-efficient communications systems with accompanying wireless phones having maximum battery life and associated standby time.
Cellular telecommunications systems, such as Code Division Multiple Access (CDMA) communications systems, are often characterized by a plurality of mobile stations (e.g. cellular telephones, mobile units, wireless telephones, or mobile phones) in communication with one or more Base Station Transceiver Subsystems (BTS""s). Signals transmitted by the mobile stations are received by a BTS and often relayed to a Mobile Switching Center (MSC) having a Base Station Controller (BSC). The MSC, in turn, routes the signal to a Public Switched Telephone Network (PSTN) or to another wireless phone. Similarly, a signal may be transmitted from the PSTN to a wireless phone via a base station or BTS and an MSC.
Wireless communications networks often employ various channels, such as paging channels and traffic channels, as disclosed in the IS-95 cellular telephone standard, to facilitate communications between a wireless phone and a BTS. Paging messages are transmitted over a paging channel by a BTS to an associated wireless phone to indicate an incoming call. Once a wireless phone detects a paging message, a sequence of service negotiation messages is transmitted between the wireless phone and an associated BTS to establish a traffic channel. A traffic channel typically supports voice and data traffic.
Conventionally, a wireless telephone continuously monitors the paging channel for pages indicative of incoming calls. The receiver of the wireless phone remains on while signal processing circuitry within the wireless phone demodulates the paging channel to determine if a page was sent. Unfortunately, the receiver draws excess power, which significantly limits phone battery life.
Systems for minimizing wireless phone power consumption are often employed in the wireless phone and/or accompanying network to extend phone battery life, i.e., standby time. To improve standby time, some newer wireless phones operate in slotted mode. In slotted mode, the receiver of the wireless phone is periodically activated in accordance with predetermined paging slots established in accordance with the IS-95 telecommunications standard. An associated BTS transmits pages during the paging slots. Wireless phone standby time is extended by periodically powering-up the receiver and demodulating the paging channel rather than continuously demodulating the full paging channel as done previously.
Unfortunately, paging channel messages are often long and require extensive processing, which increases phone power consumption and reduces battery life and associated standby time. Furthermore, the design of such systems and the associated paging channels necessitates redundant processing of the lengthy paging channel messages to detect incoming calls. This further reduces phone battery life.
Further increases in phone standby time are achieved via a relatively new addition to the IS-95 telecommunications standard known as offline processing. In a wireless communications network employing offline processing, a pair of quick paging channel (QPCH) symbols is periodically transmitted to the wireless phone. The quick paging channel symbols, i.e., quick pages, indicate the presence or absence of an incoming call to be established on a forthcoming traffic channel (F-CCCH). The QPCH symbols arrive in pairs at 9600 bits per second (bps) or 4800 bps. The time slots at which the QPCH symbols are transmitted from an associated BTS are known by the wireless phone, which periodically powers-up the receiver at corresponding time slots.
In a wireless phone employing offline processing, the wireless phone receiver powers-up, samples the QPCH, then immediately powers-down the receiver and processes the QPCH sample offline (when the receiver is off). Subsequent analysis of the QPCH sample or samples indicates whether the wireless phone should power-up the receiver and demodulate the paging channel to receive an incoming page associated with an incoming call. Use of the QCPH helps minimize receiver activation time and the instances of complete paging channel demodulation, enabling a reduction in wireless phone power consumption and an associated extension in phone battery life. Unfortunately, existing systems and methods for demodulating the QPCH and deciding whether or not to process the subsequent full paging channel based on the QPCH are undesirably large, expensive, consume excess power, and are generally inefficient. Furthermore, existing systems often fail to effectively employ both symbols of the QPCH and noise power estimates of the received signal to effectively determine whether to process the forthcoming full paging channel.
Hence, a need exists in the art for an efficient and cost effective system and method for receiving and processing quick paging channel symbols to determine whether or not to process the forthcoming full paging channel. There exists a further need for an efficient system and method that employs noise power estimates and selectively employs either one or both symbols of each quick paging channel slot, in accordance with the existing signal environment, to most efficiently and reliably detect the presence of an incoming page via minimal hardware.
The need in the art is addressed by the system for efficiently employing a quick paging channel signal to determine the presence of a forthcoming primary paging channel of the present invention. In the illustrative embodiment, the inventive system is adapted for use with a wireless communications system employing a quick paging channel and a primary paging channel. The system includes a first mechanism for selectively processing a first quick paging channel symbol and/or a second quick paging channel symbol of a received signal based on a first decision parameter and/or a second decision parameter and providing a first indication in response thereto. A second mechanism processes the first quick paging channel symbol in response to the first indication and provides a second indication in response thereto indicating whether a forthcoming primary paging channel signal should be received and processed. A third mechanism processes the second quick paging channel symbol in response to the first indication and the second indication and provides a third indication in response thereto specifying whether the primary paging channel should be received and processed.
In a specific embodiment, the system is adapted for use with a mobile station and further includes a fourth mechanism for selectively employing the third mechanism when the second indication does not indicate that the forthcoming primary paging channel should be received and processed. The first decision parameter (CSI1) is representative of a quality of a signal environment through which the received signal propagates and is described by the following equation:             CSI      1        =                  E        pilot1                              I          ^                o1              ,
where Epilot1 represents the normalized pilot energy of a portion of the pilot signal associated with the first quick paging symbol, and Îo1 represents a total energy of a portion of the received signal associated with the first quick paging symbol. The second decision parameter (D1) is described by the following equation:             D      1        =                  QP        1                    E        pilot1              ,
where QP1 is the dot product, cross product, or a combination thereof (depending on the mode of the mobile station) of the first symbol with an estimate of the pilot signal associated with the first symbol. A first comparison mechanism compares the first decision parameter to an erasure threshold and indicates (via the first indication), based on the comparison, that the first quick paging channel symbol should be processed when the first decision parameter is greater than a predetermined erasure threshold. The first comparison mechanism indicates, via the first indication, that the second quick paging symbol should be processed (and not the first quick paging symbol) when the first decision parameter is less than the erasure threshold.
The second mechanism includes a second comparison mechanism for comparing the second decision parameter (D1) to a first on-off threshold and indicating, via the second indication, that the forthcoming paging channel should not be received and processed for a forthcoming page. Another mechanism included in the second mechanism selectively places the mobile station into a sleep state in response to the second indication. The second comparison mechanism indicates, via the second indication, that the second quick paging channel symbol should be processed when the first indication does not indicate that the second quick paging channel should be immediately processed and when the second decision parameter is greater than the on-off threshold.
The third mechanism includes a mechanism for selectively computing the following decision parameter (D) in response to the mechanism for indicating:       D    =                                        QP            1                                σ            1            2                          +                              QP            2                                σ            2            2                                                            E            pilot1                                σ            1            2                          +                              E            pilot2                                σ            2            2                                ,
where "sgr"12 represents estimated summed noise power over all multipath components associated with a portion of the received signal containing the first quick paging channel symbol; "sgr"22 represents summed noise power associated with a portion of the received signal containing the second quick paging channel symbol; QP2 is the dot product, cross product, or combination thereof between signal components of the second quick paging channel symbol and the pilot signal associated with the second quick paging channel symbol; and Epilot2 represents pilot signal energy associated with the second pilot channel symbol. The third mechanism further includes a mechanism for comparing D to a second on-off threshold and indicating, via the third indication, that the forthcoming primary paging channel should be processed when D is greater than the second on-off threshold, and indicating, via the third indication that the forthcoming primary paging channel should not be processed when D is approximately less than the second on-of threshold.
The novel design of the present invention is facilitated by the first mechanism and the second mechanism, which strategically process the first quick paging channel symbol and/or the second paging channel signal as needed, which avoids sometimes unnecessary processing of the second quick paging channel symbol, yet provides for a maximum probability of successful detection of a forthcoming primary paging channel. Further reliability is obtained by selectively employing noise power estimates associated with the first and second quick paging channel symbols to compute an optimal decision metric for determining when to process a forthcoming primary page.